1. Field of the Invention
This invention relates to fluid handling systems and more particularly it concerns novel arrangements for delivering a reactive mixture, such as a multiple component thermosetting polymer, to molds for the casting of various objects. The invention is particularly well suited for the manufacture of plastic footwear such as skate and ski boots.
2. Description of the Prior Art
Plastic boots have previously been manufactured by injecting or pouring a reactive mixture of a synthetic rubber prepolymer, such as that sold under the trademark Adiprene, and a curing agent such as that sold under the trademark Moca, into molds and allowing polymerization to take place within the molds. In general synthetic rubber prepolymers and their curative were separately prepared in batch type systems and then were mixed together immediately prior to injection into the molds. Prior to mixing, however, the prepolymer must first be preheated to and maintained at a temperature of about 190.degree. F. (88.degree. C. The heated prepolymer must then be subjected to a vacuum degassing operation. The catalyst also must be preheated to a temperature of about 235.degree. F. (113.degree. C.) to bring it to a liquid state. The thus heated prepolymer and curative are then separately pumped into a common mixing device were they are throughly intermixed just before injection into a mold. It is important in the mixing operation to maintain a proper volume ratio of the prepolymer and curative. The volume ratio of these components is controlled by separately pumping each component at a predetermined rate into the mixing device.
Because of the nature of many synthetic rubber prepolymers and many curing agents, in both their unmixed and mixed condition, a number of precautions must be observed in handling them. One of the most important of these precautions is that the ratio of the prepolymer and the curative be very closely controlled. If the amount of curative is too low, the prepolymer will not have proper physical properties, and an imperfect product will result. On the other hand, if the amount of the curative is too high, the excess curative will escape into the atmosphere and may present a health hazard to workers in the area.
In the past, it was difficult to maintain a precisely controlled volume ratio of the prepolymer and curative reaction components. This was because the pumps, which controlled the volume flow rates of the prepolymer and curative to a mixing device, became subjected to substantial back pressures at the end of each mold filling operation. When the flow of mixture into the mold was completed, the pumps either had to be stopped or they had to operate under conditions of high slippage. In either case, the volume delivery rate of the pumps tended to drift from their original setting and the volume ratio of the reaction mixture components correspondingly shifted.
Another difficulty experienced in connection with the use of a prepolymer and curative reaction mixture was that the mixture tended to polymerize and solidify, to a certain extent, in the mixing device and in the conduit and valve means extending from the mixing device to the mold. Because of this it was necessary periodically to clean the mixing device and conduit with a solvent. This in turn required that the overall system be stopped until the mixing device and conduit could be disassembled, cleaned and reassembled. In the past, this disassembly and cleaning was very time consuming and tedious. Furthermore, whenever the system was stopped, it became necessary thereafter to recalibrate the pumps which supplied the reaction mixture components to the mixing device.
The mixing devices of prior art mold filling systems were complex and bulky and they had to be provided with a motor or other mechanical drive to turn a stirring element. Because of this the mixing device had to be rigidly mounted and the mold had to be positioned relative to the outlet of the mixing device in order to be fitted. This required considerable effort on the part of the operator and led to operator fatigue. Static mixers were known in the prior art as evidenced by U.S. Pat. Nos. 3,286,992 and 3,664,638. However, these mixers were not used with mold filling systems. Moreover, they did not lend themselves to rapid disassembly for cleaning purposes.